Benefits of plant diversity to ecosystems: immediate, filter and founder effects

1 It is useful to distinguish between the immediate effects of species richness on ecosystems and those which become apparent on a longer time scale, described here as filter and founder effects. 2 Relationships between plant diversity and ecosystem properties can be explored by classifying component species into three categories – dominants, subordinates and transients. Dominants recur in particular vegetation types, are relatively large, exhibit coarse-grained foraging for resources and, as individual species, make a substantial contribution to the plant biomass. Subordinates also show high fidelity of association with particular vegetation types but they are smaller in stature, forage on a more restricted scale and tend to occupy microhabitats delimited by the architecture and phenology of their associated dominants. Transients comprise a heterogeneous assortment of species of low abundance and persistence; a high proportion are juveniles of species that occur as dominants or subordinates in neighbouring ecosystems. 3 A 'mass ratio' theory proposes that immediate controls are in proportion to inputs to primary production, are determined to an overwhelming extent by the traits and functional diversity of the dominant plants and are relatively insensitive to the richness of subordinates and transients. Recent experiments support the mass ratio hypothesis and the conclusion of Huston (1997) that claims of immediate benefits of high species richness to ecosystem functions arise from misinterpretation of data. 4 Attribution of immediate control to dominants does not exclude subordinates and transients from involvement in the determination of ecosystem function and sustainability. Both are suspected to play a crucial, if intermittent, role by influencing the recruitment of dominants. Some subordinates may act as a filter influencing regeneration by dominants following major perturbations. 5 Transients originate from the seed rain and seed banks and provide an index of the pool of potential dominants and subordinates at specific sites. Where the landscape carousel operates against a background of declining diversity in the reservoir of colonizing transients, we may predict that a progressive loss of ecosystem functions will arise from the decline in the precision with which dominants can engage in the re-assembly and relocation of ecosystems.